docs: Document `vibra` module

Signed-off-by: Rahix <rahix@rahix.de>

Documentation/index.rst

@@ -23,6 +23,7 @@ Last but not least, if you want to start hacking the lower-level firmware, the
    pycardium/overview
    pycardium/color
    pycardium/leds
+   pycardium/vibra
 
 .. toctree::
    :maxdepth: 1

Documentation/pycardium/vibra.rst

+``vibra`` - Vibration Motor
+===========================
+
+.. py:function:: vibra.vibrate(millis)
+
+   Turn on the vibration motor for a certain duration.
+
+   **Example**:
+
+   .. code-block:: python
+
+      import vibra
+
+      # Turn on vibration motor for 60 ms
+      vibra.vibrate(60)
+
+   :param int millis:  Duration for the vibration motor to be on.
+
+
+.. py:function:: vibra.set(state)
+
+   Permanently set the state of the vibration motor to either *on* or *off*.
+
+   .. warning::
+
+      If your code for some reason crashes between turning the motor on and
+      back off again, the motor will continue running.  Consider using
+      :py:func:`vibra.vibrate` instead.
+
+   :param bool state: ``True`` to turn on, ``False`` to turn the motor off.